It seems like Sally might have finally banished Donna for good on Being Human Season 4 Episode 3, as she discovered by using the spell book that she was actually bringing the two of them closer.
For once, though, Sally's anger was so great she set everyone ablaze, absorbed the spells and Donna disappeared.
That seemed almost as easy as crawling out of Purgatory, now that I think of it. Donna will probably make her way back somehow. There's no doubt, however, that the more spells Sally ingests the more powerful she becomes.